Comics older than myself that I have read are all Aquaman-related. I've read his entire Silver Age appearances and Bronze Age appearances outside of Justice League which pre-dates me since I was born in 1987. That's the only character I've gone that far back with. Debated about doing the same with Hawkman but haven't yet. Oh, and I've read like the first 25 issues of X-Men because I have that big soft trade paperback of the first 25 issues. I also have a trade with much of the early Thor stuff such as his first appearance but haven't gotten around to reading the books yet.
I enjoyed the really old Aquaman tales because of the understanding that it was written for a younger age. Also it is hard not to love the art of some of it like Ramona Fradon.